1. Field of the Invention
The present invention generally relates to a test key structure and a method for measuring a step height by using such test key structure. In particular, the present invention is directed to a test key structure for measuring electrical resistances and a method for measuring a step height by using such test key structure.
2. Description of the Prior Art
A smooth and flat silicon wafer is the basis for fabricating integrated circuits. Difference of levels formed by the elements in the silicon wafer, such as the height difference of the shallow trench isolation (STI) in the MOSFET adjacent to the active region, causes the uneven surface of the substrate, which is called “a step height.” Such step height is critical to the quality of the semiconductor process.
This critical factor has influences in many aspects. For example, with respect to processes, the unevenness of the surface of the substrate is critical to the lithographic precision. Failure of the lithographic precision may cause defocus or distortion. With respect to elemental characters, the reliability of the element is influenced, too. For example, the evenness of the surface of the substrate is highly related to the junction leakage and other effects, such as the short channel effect (SCE) or the narrow width effect (NWE). As a result, in order to have a steady process control, such as for the chemical mechanical polishing or a lithographic procedure, a quick method to acquire the step height of the substrate without damaging the substrate is necessary.
There are certain known methods to acquire the step height of the substrate. For example, an atomic force microscope (AFM) may be used to observe the situation of the surface of the substrate. However, it is difficult to cover a sufficient surface within a limited time period.
Another method may also acquire the step height of the substrate, which is called a transmission electron microscopy (TEM). Albeit a transmission electron microscopy is more time-saving compared with the atomic force microscope, it is needed to destroy the substrate to acquire the sample. In light of no current method being able to acquire the step height of the substrate in a both quick and non-destructive way, the current solutions to acquire the step height of the substrate are still needed to be improved.